Fluorine-containing polymers (i.e., fluoropolymers or fluorinated polymers) are a commercially important class of materials. Many fluoropolymers are known to exhibit high thermal stability and usefulness at high temperatures, as well as extreme toughness and flexibility at very low temperatures. Many fluoropolymers are also almost totally insoluble in a wide variety of organic solvents, and resistant to many chemical compounds that might degrade other classes of less resistant materials. Additionally, many fluoropolymers are recognized for their barrier properties; i.e., their ability to prevent the passage of liquids or vapors.
Increased concerns with evaporative fuel standards have led to a need for fuel system components that have improved barrier properties, to minimize the permeation of fuel vapors through automotive components such as fuel filler lines, fuel supply lines, fuel tanks, and other components of a automobile emission control systems. Multi-layer articles such as multi-layer tubing have been used within these applications. These multi-layer products can comprise a fluorinated layer, which provides an inert, vapor barrier, and one or more other layers that can add strength, rigidity, or other mechanical properties to the multi-layer article. As an example, some multi-layer articles include a fluorinated layer and a layer comprising a substantially non-fluorinated polymer.
To be most useful, these multi-layer articles should not delaminate during use. That is, the adhesive bond strength between the different layers of the multi-layer article should be sufficient to prevent the different layers from separating. A variety of methods have been employed to increase the adhesive bond strength between a layer comprising a fluoropolymer and a layer comprising a substantially non-fluorinated polymer. For example, a layer of adhesive can be added between the two layers. As an alternative to or in addition to adhesives, surface treatment of one or both of the layers has been used to increase the adhesive bond strength between the two types of layers. For example, layers comprising a fluoropolymer have been treated with charged gaseous atmosphere followed by application of a layer of thermoplastic polyamide. As another approach, "tie-layers" have been used to increase the adhesive bond strength between a fluoropolymer and layer comprising a substantially non-fluorinated polymer. A tie layer is a layer comprising a blend of materials from two dissimilar layers disposed between the two dissimilar layers. See e.g., European Patent Application 0523644 (Kawashima et al.).
The adhesion between a substantially non-fluorinated polymer and a fluoropolymer, wherein the fluoropolymer is derived from vinylidene fluoride (VDF) and hexafluoropropylene (HFP), has been found to increase upon exposure of the fluoropolymer to an amine compound. Fluoropolymers derived from VDF and HFP are relatively susceptible to attack by basic materials due to the presence of an electronegative, carbon-bonded, electron-withdrawing --CF.sub.3 group (from the HFP) adjacent to a carbon-bonded hydrogen atom provided by the VDF monomer. Amines can therefore be reacted with the VDF/HFP fluoropolymer to increase adhesion of a fluoropolymer to a substantially non-fluorinated substrate.
On the other hand, fluoropolymers derived from fluorinated monomers that include substantially no VDF are known to exhibit superior inertness relative to fluoropolymers derived from VDF monomers, and are consequently more resistant to chemical attack. These fluoropolymers are expected to be inert from attack by a base, and are therefore less apt to undergo adhesion-promoting reactions with amines. Examples of such fluoropolymers include fluoropolymers derived from monomers comprising tetrafluoroethylene (TFE) and/or hexafluoropropylene (HFP), and substantially no vinylidene fluoride monomers.
What is needed, but not provided by the prior art is a method of increasing the adhesive bond strength between a substantially non-fluorinated polymeric material and a fluoropolymer, the fluoropolymer being derived from one or more of TFE and HFP monomers, and one or more non-fluorinated olefinically unsaturated monomers, but substantially no vinylidene fluoride monomer.